When many fruits and vegetables are subjected to disorganization of their tissue structure, e.g. by peeling, cutting or coring, the surfaces thereby cut often suffer undesirable changes, particularly the development of a dark and unappealing color usually termed "browning". Changes such as browning are thought to be due to certain chemical changes in the plant tissue which are catalyzed by enzymes present in the tissue such as polyphenoloxidase. When the plant tissue structure is disorganized, such as by cutting operations, the natural segregation of chemically reactive compounds in the tissue from enzymes in the tissue is destroyed, allowing the enzymes to contact the reactive compounds. This results in the rapid onset of irreversible chemical changes evidenced by undesirable browning. The browning of plant tissue is usually attributed to a mechanism whereby orthodihydroxy phenolic compounds such as catechol, caffeic acid, chlorogenic acid, or gallic acid, present in the plant tissue are oxidized by enzyme catalyzed reactions into orthoquinones which then polymerize to form the dark colored substances which are visible as browning.
Browning may be prevented or retarded by treatment of the cut plant tissue using one or more of the following techniques:
1. Removal or exclusion of oxygen, either chemically or physically;
2. Alteration of the catechol structure which is subject to enzymatic oxidation;
3. Application of a chemical to the tissue to reduce the quinone structure back to the catachol structure; or
4. The deactivation or inhibition of the enzyme polyphenoloxidase.
It is known in the art to use ascorbic acid in attempting to reduce the quinone structure back to the catachol structure for a period of time sufficient to allow the natural enzymes such as polyphenoloxidase to become inactivated by natural breakdown. Unfortunately, it has been found that, if sufficient ascorbic acid is employed to perform this function and to allow the enzymes to become inactive, the resulting fruit or vegetable product is very acidic and acquires an unpleasant and unmarketable taste. Further, the natural colors in the plant tissue may be adversely affected. These deficiencies in using ascorbic acid to retard browning are particularly apparent when the treated fruit or vegetable is subsequently stored under aerobic conditions. Aerobic storage has generally been found to cause greater and more rapid deterioration than anaerobic.
One of the primary reasons why ascorbic acid has been thought to be an effective anti-browning agent, except for the undesirable acidic qualities and color loss which it imparts to the treated fruit or vegetable, is that, being an acid, the ascorbic acid tends to produce a relatively low pH condition and to render the treated vegetable tissues quite acidic. This has been thought to control browning, since the chemical activity of enzymes such as polyphenoloxidase has been found to be highest between a pH of about 5.9 - 6.3. Thus, when ascorbic acid has been employed to provide a pH below the 5.9 level, for example, the activity of the browning enzymes has been thought to be reduced thereby.